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GM500 Relay Reloaded

Image of GREG SPEARS. GREG SPEARS looks at further uses for the Universal Relay Switch in the Gaugemaster Electrics range.


In this article my technical team and I will be revisiting the GM500 Universal Relay Switch and some of the various ways that it can enhance your layout.

What is the GM500 Universal Relay?

The GM500 is a self latching relay that only requires a small pulse of power to switch from side to side, and unlike traditional relays, will stay latched without the need for constant power. To connect the relay, users can simply wire it in parallel with the point motor feed. This can be done with an AC or common negative DC supply. Users with common positive (as typically found on DCC accessory decoders) can use the GM500D Universal Relay Switch (DCC Friendly) a version created specifically for use with DCC. The switch side of the relay is a double pole/double throw set up with an input range of 0-30v @ 2Amp max. Typical uses are for frog polarity and signal control, so if you need to add or upgrade your point motor switch, the GM500 range is what you need. Below is a diagram of a typical setup - the diagram has a DC supply for reference but as mentioned AC is also fine. Full instructions can be viewed HERE .


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A useful feature of the switch for both DC and DCC users is frog polarity on a double slip as seen below. The diagram features a GMC-CDU Capacitor Discharge Unit (CDU) which is commonly used when multiple motors are being active on a single switch.

GM500 Double Slip.

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Shuttle System

Now we have seen some typical ways to use the GM500 around the layout, let’s now look at some simple ways to add some small features. Below is a diagram of a basic shuttle unit using the GM500 to reverse the polarity. The first example shows the track feed going via the GM500 with reed switch activation. A small magnet has to be added under the loco or wagons to activate the GM99 reed switch which can be purchased as a pack.

This diagram shows a 16v AC supply which could come direct from your controller if it has a suitable output, or a separate supply like our GMC-WM1 Wall Mounted Transformer. This setup is useful as there is no time limit from end to end, so slow moving models can be run as slow as you wish. The second diagram shows one of the reed switches being replaced with a push to make switch and a diode. The diode stops the flow in one direction as the model crosses the break in the track. The locomotive will now wait until the switch is activated and allow travel to the reed switch and back. This is particularly useful for displays where constant running is not required or just as a bit of fun for small fingers that are built for fun rather than happy operation! Push buttons and diodes can be found HERE and HERE.

GM500 Shuttle Unit.

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Station Stop System

In keeping with the last idea, you can also incorporate this into a loop as the GM500 is simply turning a section of track on and off. A push button to makes the track section live and a reed switch to switch it back off using items we have already mentioned. Some people might even want to experiment with section control but we will save that for another time...


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Automating Point Motors

So, we have covered switching frogs and signals - as well as reversing polarity and track sections, but there is one thing we get asked for on a regular basis: "how can I automate a point motor?". You will be pleased to hear the GM500 can do that as well! Below are some diagrams showing how the GM500 via reed activation not only supplies power to the motor but also to itself, creating a self cancellation mode. This can be used on single or multiple motors as shown and incorporating a second GM500 gives you full control in both directions.

GM500 single-PULSE.

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GM500 single-PULSE-x2.

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GM500 Dual-Pulse-x2.

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Just a couple of things to remember when using this - the supply is a constant AC and not a pulse as seen in CDU. This is to ensure a constant reliable power supply to both motor and the GM500. Also if you expand the picture you will notice the RESISTOR BYPASS. The GM500 has a 1K resistor built in to protect the relay coils and limit current draw on any switching devices, especially low power reed switches etc. However the power consumption of the motor(s) can starve the GM500 with the standard resistor in place, so in some situations this will need to be bypassed to allow sufficient power to operate the relay. This might not be required depending on the voltage and the internal resistance of the motors being used. For reference, we used our WM1 and GMC-PM10 motors. When bypassing the resistor you can either run a small wire link from pad to pad or attach your wire directly to the second pad as shown below.


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Reverse Loops

So if we put automatic polarity control and point motor operation together you could make a reverse loop module. The diagram below shows a double continuous loop with a model GMC-COMBI Single Track Controller controlling the loco and supplying the AC for the GM500 controlling the reverse polarity section. For the point motors and power we used a WM1 and GMC-PM10 motors mounted on top for clarity but obviously under board is also fine. We used standard insulfrog points just to keep it simple to follow.


Click image above to view/download full-size PDF image.

So, the last thing to do was to build a layout and run it continuously for 8 hours (Editors note – we do actually do other stuff than play trains here, honestly...) At 7-8 seconds a lap, we clocked up over 3,500 laps with some parts operating twice per lap, to see it in action, here’s a short video.

Product Links

  • GM500 Universal Relay Switch
  • BPGM500 Universal Relay Switch (3 Pack)
  • GM500D Universal Relay Switch (DCC Friendly)
  • GMC-CDU Capacitor Discharge Unit
  • GMC-WM1 Wall Mounted Transformer - 1x 16v AC or 12v DC @ 1.1A
  • GM516 Push to Make Switch Pack - Red (5)
  • GM74 1 Amp Diodes for PCU1 & SS1 (20)
  • GMC-PM10 Classic Solenoid Point Motor
  • GMC-COMBI Single Track Controller with Plug in Transformer

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